1. Field of the Invention
The present invention relates generally to angular position measurement devices, and in particular to a low cost slip ring encoder for determining the angular position of a satellite solar wing drive shaft.
2. Description of the Related Art
Commercial and military satellites generate electricity while in-orbit to operate satellite subsystems. Typically, this power is provided, at least in part, by solar arrays, also known as solar wings. On-orbit operation of such satellites typically require precision angular orientation of the solar wings to maximize exposure to solar energy, to minimize orbital perturbations from solar wind, and to minimize the effects of thruster impingement on the solar arrays.
In current satellite designs, solar wing position information is provided by potentiometers used in conjunction with the satellite solar wing drive (SWD). Such potentiometers are usually mounted to the solar wing shaft, and provide a resistance that varies according to the shaft position.
Unfortunately, while the foregoing potentiometers provide continuous accurate position feedback information, they are prone to on-orbit failures. Other position feedback devices, such as resolvers or optical encoders are typically more reliable, but like the potentiometers, these devices add unnecessary complexity, weight and cost to the spacecraft.
There is therefore a need for a reliable, low cost, simple device and method for measuring the angular position of a solar wing. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method, apparatus, and an article of manufacture for determining the angular position of a shaft. The apparatus comprises a circumferentially alternating series of conducting and substantially non-conducting segments on an exterior surface of the shaft, and at least one contact surface in communication with the surface of the shaft, the contact surface sized to communicate with no more than one conducting segment at a time. Several embodiments of the foregoing apparatus are possible. For example, in one embodiment, two electrically isolated or redundant contact surfaces are used to communicate with different surfaces of the shaft to assure that at least one of the contact surfaces is in electrical communication with at least one of the conducting segments at any shaft angular position. In another embodiment of the invention, the apparatus comprises a second circumferentially alternating series of conducting and substantially non-conducting segments on the exterior surface of the shaft, and a second contact surface in communication with the surface of the shaft. The first and second series of conducting and non-conducting segments can be arranged to provide additional angular resolution of the encoder.
Another embodiment of the present invention is described by a method of determining the angular position of a shaft by sensing a conductivity of an exterior surface of the shaft having an alternating series of conducting and substantially non-conductive surfaces, and determining the shaft angular position using the sensed conductivity of the exterior surface of the shaft. In one embodiment of the invention, the foregoing conductivity is used to supply datum information, and the precise angular position of the shaft is determined from the datum and from a number of stepper motor step commands supplied to rotate the shaft in operation. The present invention can also be used in an embodiment in which the angular position of the shaft is determined solely from the number of stepper motor commands provided to rotate the shaft. In this embodiment, sensed conductivity is periodically used to confirm the angular position determined from stepper motor commands.
The foregoing provides information regarding the shaft position with greater reliability, less cost, lower weight, and within a smaller physical envelope than previous designs.